The casing according to the present invention can be used wherever a casing for power driven chain saws is to be constructed. In this case the invention relates to a casing for a power driven chain saw with a drive unit driving the chain revolvingly via the saw rail with a rail supporting means consisting of two fastening bolts penetrating a slot-like aperture provided in the saw rail and a saw chain tightening device arranged within the area of the rail supporting means as well as a sprocket wheel cover overlapping and covering the rail supporting means that can be secured to the casing.
In the known power driven chain saws a casing divided into two or more portions is used, in which a drive unit, by way of example, a combustion engine or an electromotor, is disposed which drives the saw chain revolvingly via the saw rail by means of a sprocket wheel. In this case, the saw rail, which is also referred to as "saw sword", is provided with a slot-like aperture into which penetrate two fastening bolts arranged on the casing. The saw rail is secured by means of retaining nuts screwed onto the fastening bolts.
In order to tighten the saw chain, the saw rail is displaced. This is done by loosening the saw rail fastening screws and by actuating a saw chain tightening device for displacing the saw rail, as a consequence of which the fastening bolts are displaced within the slot-like aperture. In power driven chain saws of this type the saw rail retaining nuts serve at the same time for securing the sprocket wheel cover overlapping and covering the sprocket wheel and the rail supporting means. If the chain is to be replaced now, It will be necessary not merely to loosen the rail retaining nuts, but to screw them off completely from the fastening bolts in order to be able to remove the sprocket wheel cover. In the course of this operation there exists the risk, particularly when working in forests, of the fastening nuts being lost. In addition, the handling of such a power driven chain saw is exceedingly complicated when replacing the chain since, once the sprocket wheel is removed, the saw rail, too, is no longer attached and is held loosely on the fastening bolt, so that it is possible only with difficulties to arrange the saw chain around the saw rail. It would therefore be desirable for the saw rail to be already mounted relatively firmly, but adjustably and with fitted retaining nuts on the casing when the saw chain is placed in position. Over and above that, the disadvantage exists in the known saws that, when the saw chain is placed inaccurately upon the saw rail and the clamping screws are subsequently tightened, the chain tightening device frequently is destroyed.
When adjusting the chain tension it has been necessary up till now to actuate a chain tightening device which, when seen from the assembly side, is located behind the saw rail. If, when placing the saw chain in position, it is now intended to bring about a change in the position of the saw rail, it will be necessary to grip the saw rail from behind, which entails a risk of injury because the engagement claws of the power driven chain saw are often arranged behind the saw rail.
In a construction of a power driven chain saw which has become known, the chain tightening device is intended to be actuated through the sprocket wheel cover from the assembly side. However, also in this chain saw the sprocket wheel cover is already mounted when the tightening device can be actuated. A saw chain tightening device which can be actuated without difficulties when the sprocket wheel cover is removed is not known up to the present.